1. Field of the Invention
The present invention relates to the field of logging and inspecting of oil, gas and mineral wells. More particularly, the invention relates to a multi-frequency array induction tool.
2. Description of the Related Art
Induction logging tools are instruments used in logging operation in boreholes that are drilled into underground rock formations in the search for oil, gas, or minerals. Induction logging tools measure the electrical conductivity of rock formations to determine the presence and the amount of desired minerals in a particular pay-zone. Oil and natural gas cause the rock to have a lower than usual conductivity because these fluids are electrically non-conducting and they displace connate fluids such as conductive saline water. Induction logging tools ideally provide accurate quantitative measures of the fractional saturation of oil or gas in the pay zone.
Induction logging tools employ arrays of sensors that map the rock conductivity at various radical distances from the borehole so that the perturbing influence of invasion of borehole fluids may be reduced.
The tools operate on the principle of induced eddy-currents, also known as Foucault currents, that are substantially proportional to conductivity and which may be excited and detected using sensitive coils. Tools that are known in the art use arrays of coils that provide capabilities of sensing conductivity to one or more different radial depths.
Despite advances in induction logging tools, several problems with the tools remain to be solved. For example, induction tools typically report errors due to the influence of adjacent rock-beds of contrasting conductivity. Further, boreholes may hold conductive fluids that influence readings of an induction logging tool. The influence of the conductive fluids increases when the borehole diameter varies due to caving or when the cross-section is not circular due to various drilling problems. Moreover, borehole fluids containing slightly magnetic materials introduce more subtle problems.
Highly conductive formations present further problems for induction logging tools due to xe2x80x9cskin effect.xe2x80x9d Skin effect causes a loss of proportionality between a received signal and formation conductivity, thereby making interpretation of signals from induction logging tools more complex. Skin effect prevents operators from neglecting attenuation of propagated signals in formations that are highly conductive. Typically, induction logging tools allow for a moderate skin effect at higher conductivity and correct for the skin effect. However, responses in highly conductive formations are often non-linear. Corrections for non-linear responses are difficult to make. Conversely, very low conductivity rocks present accuracy problems for logging tools due to low signal to noise ratios.
Logging tools that traverse sequences of thin rock-beds or boundaries with high relative dip angles present spurious responses that are difficult to correct. Further, logging tools do not accurately log boreholes that are invaded by borehole fluids that have conductivity very different from the connate fluids in the rock, or that create complex annulus profiles in the invaded zone.
Several attempts have been made to try to resolve these problems, each of which provided complex and expensive tools, and each of which exhibits some shortcoming in their ability to accurately measure and profile the rock conductivity over a wide range of radial distances in conditions that are commonly encountered in oil explorations. All of these designs employ xe2x80x9cdepth-shiftingxe2x80x9d of the recorded data, to align the effective measurement points of the individual measurements, with resulting errors when the tool motion is erratic.
In accordance with the present invention, an apparatus and method is disclosed for induction logging of electrical properties of earth formations that operates at low frequencies while remaining resistant to skin effect and effect and maintaining mutual balancing. The apparatus includes a plurality of transmitter coils that are at a plurality of distances from a measure point located at an end of the apparatus, and a receiver coil array coupled to receive induced voltages resulting from currents induced in the earth formations by one or more transmitters of the plurality of transmitter coils, wherein the measure point is located within the receiver coil array, wherein one or more of the plurality of distances from the measure point are determined according to a function of one or more frequencies associated with one or more transmitters of the plurality of transmitters.
The method includes providing a tool operable in a borehole, the tool including a plurality of transmitter coils that are at a plurality of distances from a measure point located within a receiver coil array at an end of the tool. The method further includes receiving at the receiver coil array induced voltages resulting from currents induced in the earth formations by one or more transmitters of the plurality of transmitter coils and using the energy from the plurality of transmitter coils to determine the electrical properties of earth formations. The method further includes choosing one or more of the plurality of distances from the measure point according to a function of one or more frequencies associated with one or more transmitters of the plurality of transmitters.
According to an embodiment, the function includes determining the distance that is inversely proportional to a square root of the frequency associated with the one or more transmitters. The distances include {square root over (2)}*N, {square root over (2)}/2*N, {square root over (2)}/4*N, 2N, N, N/2 and N/4, wherein N is a fixed distance such as one meter. The method further includes subtracting signals received from a borehole compensation array disposed at the end of the tool from signals received at the receiver coil array, the subtracting reducing the influence of local changes in borehole diameter. According to an embodiment, the borehole compensation array includes a main borehole compensation receiver coil, a borehole compensation bucking receiver coil coupled to the main borehole compensation receiver coil, and a borehole compensation transmitter coil configured to be operable with the main borehole compensation receiver coil and the borehole compensation bucking receiver coil.
An embodiment of the method includes receiving residual skin-effect error voltages at the receiver coil array that are substantially the same for each transmitter of the plurality of transmitters. Further, an embodiment provides that the plurality of transmitter coils have the same axial spatial relationship. An apparatus and method provides that at least one transmitter of the plurality of transmitters contains a magnetic core material to enhance the magnetic moment of the at least one transmitter, the magnetic moment of the transmitter being increased independent of mutual balancing of the transmitter-receiver combination of the at least one transmitter and the receiver coil array. The enhancing of the magnetic moment of the at least one of the transmitters of the plurality of transmitters permits operations at frequencies of 8 kHz and lower. Further the receiver coil array, according to one embodiment, is coupled to one or more receiver circuits located near the receiver coil array, the location assisting in maintaining the mutual balancing of the transmitter-receiver combination.
An embodiment of the apparatus and method includes a transmitting circuit energizing the plurality of transmitter coils via a master crystal-controlled oscillator coupled to a binary divider string to provide a plurality of frequency signals, and a receiving circuit coupled to the transmitting circuit. The transmitting circuit includes a plurality of band-pass filters coupled to receive the plurality of frequency signals; and a plurality of power amplifiers coupled to the plurality of transmitter coils, the plurality of transmitter coils further being coupled to a plurality of capacitors, each transmitter coil having an associated capacitor for tuning the transmitter coil. The transmitting circuit further includes a plurality of phase-sensitive detectors providing reference inputs of the receiving circuit. The receiving circuit includes a mutual balancing system for balancing received signals associated with each transmitter coil of the plurality of transmitter coils. The mutual balancing system further includes a receiving coil, a bucking coil, and one or more band-pass filters coupled to the receiving coil at one or more tap locations, the tapping at different turns counts of the receiver coil to provide mutual balancing for associated transmitter coils.
In an embodiment, the receiving circuit includes one or more phase-sensitive detectors synchronized with one or more transmitters to demodulate voltages received by the receiver coil, the balancing system and the phase-sensitive detectors enabling simultaneous measurements of the earth formations independent of mutual interference.
In another embodiment, the apparatus and method provides that the tool includes a control circuit coupled to the transmitting circuit and the receiving circuit, the transmitting circuit further including one more multi-pole switches configured to sequentially couple the plurality of transmitter coils to a plurality of frequency signals, the one or more multi-pole switches coupling the plurality of frequency signals to one or more band-pass filters. Further, the control circuit coordinates which of the frequency signals is chosen by the one or more multi-pole switches and chooses one or more of a plurality of tapped signals. In one embodiment, the control circuit is implemented as a software program in a micro-controller circuit, the software program generating control signals for the one or more multi-pole switches.